J.Y. Pastor

Stress Intensity factor, compliance and CMOD for a General Three-Point-Bend Beam

New simple and general expressions for the stress intensity factor, compliance and crack mouth opening displacement for three-point bend specimens are computed. Inverse functions giving the crack length as a function of load-point displacement or crack mouth opening displacement are also included. The expressions are valid for any crack length and for any span-to-depth ratio larger than 2.5. The expressions are checked by comparing them to direct finite element computations and to available expressions by other authors. The accuracy of the new expression is equal to or better than available formulas when compared with finite element computations, and its range of applicability is much larger. Moreover, all the new expressions exhibit the correct asymptotic behaviour for very shallow and very deep cracks.

Mechanical properties of directionally solidified Al2O3–ZrO2(Y2O3) eutectics

Abstract The relationship between microstructure and mechanical properties was studied in Al2O3–ZrO2 eutectic rods. The material, produced by directional solidification using the laser-heated float zone method, was formed mainly of colonies consisting of a fine interpenetrating or ordered network of ZrO2 and α-Al2O3 surrounded by a thick boundary region that contained pores and other defects. The flexure strength of the eutectic rods was excellent (>1.1 GPa) owing to the small critical defect size and the high toughness (7.8 MPa m ). No microstructural changes were observed after about 1 h of exposure at 1700 K, and the eutectic oxide maintained a very high strength up to this temperature. The nature of the critical defects that led to fracture, the toughening micromechanisms, and the differences between the longitudinal and transverse strength are discussed in the light of the microstructural features of the material.

Microstructure and physical properties of some oxide eutectic composites processed by directional solidification

Abstract Eutectic composites of lamellar ZrO 2 –CaO and ZrO 2 –NiO and fibrous Al 2 O 3 –ZrO 2 , Al 2 O 3 –ZrO 2 (Y 2 O 3 ), ZrO 2 –MgO and CaF 2 –MgO wide gap materials have been grown from the melt by unidirectional solidification using laser floating zone and Bridgman techniques. The unique microstructure and interface morphology of these composites led to some remarkable mechanical (strength and toughness), optical (light guiding) and transport (ionic conduction) properties. The underlying relationships between microstructure and properties were briefly discussed in each case in the light of some possible applications of these eutectics.

Influence of lactose addition to gentamicin-loaded acrylic bone cement on the kinetics of release of the antibiotic and the cement properties.

The purpose of this study was to characterize a poly(methyl methacrylate) bone cement that was loaded with the antibiotic gentamicin sulphate (GS) and lactose, which served to modulate the release of GS from cement specimens. The release of GS when the cement specimens were immersed in phosphate-buffered saline at 37 degrees Celsius was determined spectrophotometrically. The microstructure, porosity, density, tensile properties and flexural properties of the cements were determined before and after release of GS. A kinetics model of the release of GS from the cement that involved a coupled mechanism based on dissolution/diffusion processes and an initial burst effect was proposed. Dissolution assay results showed that drug elution was controlled by a diffusion mechanism which can be modulated by lactose addition. Density values and mechanical properties (tensile strength, flexural strength, elastic modulus and fracture toughness) were reduced by the increased porosity resulting from lactose addition, but maintained acceptable values for the structural functions of bone cement. The present results suggest that lactose-modified, gentamicin-loaded acrylic bone cements are potential candidates for use in various orthopaedic and dental applications.

Strength and fracture toughness of hot-pressed bulk Bi2Sr2Ca2Cu3Ox and Bi2Sr2Ca2Cu3Ox/Ag at 77 and 300 K

Abstract A study was made of the effect of temperature (300 and 77 K) and orientation on the mechanical properties of bulk BSCCO 2223 and BSCCO 2223/Ag composites processed by hot-pressing. The unreinforced material was strongly textured and almost fully dense and its flexure strength was outstanding. The flexure strength of the composites showed the balance between the benefits due to the presence of Ag, and the harmful impact of higher porosity and reduced texture. The strength of all the materials was fairly independent of the orientation and temperature, although a noticeable improvement (6–20%) was found at 77 K when the crack propagation direction was parallel to the hot-press axis. The fracture toughness was mainly a function of the porosity with the dense, BSCCO 2223 material showing the highest value. In addition, the toughness was largely influenced by orientation and temperature, again reaching maximum (by up to 70%) in the specimens tested at 77 K with the crack plane parallel to the hot-press axis. This behavior was explained in terms of the crack deflection and of the toughening effect of ice.

In situ observation of damage nucleation in graphite and carbon/carbon composites

Abstract The flexure strength and the fracture toughness at 300 K and 77 K were measured in two isotropic polycrystalline graphites with very different microstructure and in one carbon/carbon composite. In addition, the micromechanisms of damage initiation at the notch tip were examined in situ during the fracture tests through a long focal distance microscope. It was found that the mechanical response of carbon-based materials was insensitive to the effect of cryogenic temperatures. In graphite with coarse microstructure, cracks appeared at very low stresses in various points of an ample region surrounding the notch tip, and damage progressed by their stable crack growth and link up. On the contrary, damage was localized at the notch root in graphite with a fine microstructure. High stresses were necessary to nucleate a single crack, which grew unstably, leading to immediate specimen failure. Damage in carbon/carbon composites was nucleated in the form of matrix cracks around the notch tip, but fiber yarns impeded the crack propagation until the load had increased significantly. This process was repeated several times, leading to a serrated load–deflection curve and to a marked increase in the overall fracture resistance.

Manufacturing of self-passivating tungsten based alloys by different powder metallurgical routes

Self-passivating tungsten based alloys will provide a major safety advantage compared to pure tungsten when used as first wall armor of future fusion reactors, due to the formation of a protective oxide layer which prevents the formation of volatile and radioactive WO3 in case of a loss of coolant accident with simultaneous air ingress. Bulk WCr10Ti2 alloys were manufactured by two different powder metallurgical routes: (1) mechanical alloying (MA) followed by hot isostatic pressing (HIP) of metallic capsules, and (2) MA, compaction, pressureless sintering in H2 and subsequent HIPing without encapsulation. Both routes resulted in fully dense materials with homogeneous microstructure and grain sizes of 300 nm and 1 μm, respectively. The content of impurities remained unchanged after HIP, but it increased after sintering due to binder residue. It was not possible to produce large samples by route (2) due to difficulties in the uniaxial compaction stage. Flexural strength and fracture toughness measured on samples produced by route (1) revealed a ductile-to-brittle-transition temperature (DBTT) of about 950 °C. The strength increased from room temperature to 800 °C, decreasing significantly in the plastic region. An increase of fracture toughness is observed around the DBTT.

Effect of the axial stress and the magnetic field on the critical current and the electric resistance of the joints between HTS coated conductors

High temperature superconducting (HTS) wires require a detailed characterization of the possible degradation of their properties by handling at room temperature as well as during their service life, establishing the limits for associated functional devices and systems. In this paper, we study the mechanical behavior of spliced joints between commercial HTS coated conductors based on YBCO at room (300 K) and service temperatures (77 K). Single lap shear tests were performed and the evolution of the critical current and electric resistivity of the joints were measured. The complete strain field for the tape and joints was also obtained by digital image correlation. In addition, tensile tests under an external magnetic field were performed, and the effect of the applied field on the critical current and electric resistivity of the joints were studied. Finally, finite element simulations were employed to reproduce the distribution of the stress field developed in the spliced joint samples during axial loading.

Mechanical characterization of GdBCO/Ag and YBCO single grains fabricated by top-seeded melt growth at 77 and 300 K

YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.

Effect of thermal cycling on the strength and superconducting properties of laser floating zone textured Bi-2212 rods

Abstract The influence of the thermal cycling on the flexure strength and superconducting properties of textured Bi 2 Sr 2 CaCu 2 O 8+ δ rods processed by the laser-heated floating zone method was studied. Two different temperature ranges (77–300 and 77–393 K) were used to separate the contributions of chemical reactions with liquid water from those due to the thermo-elastic stresses. The rods showed excellent resistance to thermal cycling in both cases and neither the superconducting nor the mechanical properties were significantly affected. In particular, the variation of the critical current at 77 K was below ±4% after 50 thermal cycles. The flexure strength of rods increased or decreased by ≈10% after a few tens of thermal cycles, and crack deflection along the rod axis was observed in the specimens which showed a reduction in strength. This decrease was transitory, and the flexure strength after 50 thermal cycles was similar to that measured on the as-received rods.